In recent years, in consideration of demand of society, such as environmental protection and nature conservation, research has been actively conducted on a fuel cell, which is an efficient and clean energy source. In particular, a polymer electrolyte fuel cell (PEFC) and a hydrogen internal combustion engine, which operate at low temperature, have high output, and are high in density, are expected to be used in homes or be mounted on vehicles. However, since these energy sources use hydrogen as fuel, they require a sensor for detecting leakage of hydrogen.
Thus, a sensor for detecting the concentration of inflammable gas such as hydrogen has a structure in which a sensor element is housed in a resin case with a gas permeable member and measures the concentration of gas flowing in and out through a gas hole. A known case member serving as such a resin case has a structure in which an annular groove is formed around a gas hole (opening) by insert molding using a slide mold, and a circumferential portion of a gas permeable porous membrane is embedded in resin around the gas hole and in the vicinity of the annular groove, whereby the gas hole is covered with the gas permeable porous membrane (see Patent Document 1).